This invention pertains to video signal processing apparatus and, more particularly, to a phase alternating line (PAL) video signal processing apparatus with improved chrominance subsampling consistency between frames in the absence of delay line averaging.
A video signal for broadcast television (TV) reception conventionally comprises luminance information Y and chrominance information having color difference components U and V (B-Y and R-Y respectively). U represents saturation and V represents hue. The video signal also comprises a reference or color burst to permit broadcast television receivers to decode the chrominance information. In broadcast TV receivers, a phase error can be introduced between the color burst and the chrominance information by, for example, variations in transistor input impedances. The phase error can, if uncorrected, degrade picture quality at the receiver.
In PAL TV systems, the effect of the phase error on the decoded chrominance information is reduced by inverting the V component on alternate field lines. Thus, the phase error, if constant over two successive field lines, deflects the hue of one line in one direction and the hue of the other line in the opposite direction by a corresponding amount.
A typical PAL TV receiver comprises a PAL color decoder having a delay line of one line period so that the chrominance element transmitted during field line n can be retained and averaged with that on field line n+1. This cancels the effect of the phase error in the received video signal. Each frame displayed on a TV monitor comprises two interlaced fields of lines. The delay line averaging circuitry effectively smears color in each frame spatially over groups of four lines. Therefore, the delay line reduces the vertical chrominance bandwidth of the receiver.
PAL video capture adapters for computer systems and the like usually comprise a chrominance subsampler for subsampling the U and V components of a decoded PAL video signal to reduce memory requirements and processing time. In practice, a factor of 8:1 reduction in chrominance information will still produce acceptable picture quality. The chrominance subsampler usually extracts chrominance information from every n.sup.th pixel of every m.sup.th line of a frame where n and m are integers greater than 1 and usually less than 10. However, because TV picture frames conventionally contain an odd number of lines (625 for example), the V component corresponding to a line in one frame has a different polarity to the V component corresponding to the same line in the next frame. Therefore, without the aforementioned delay line averaging, the chrominance subsampler samples V components of different polarities from one frame to the next. The phase error produces undesirable color flashing between successive frames, particularly in highly saturated reds. The color flashing is produced by beating between the subsampling frequency and the four field phase alternation pattern of the PAL video signal.